base64: base64url encoding has no padding

base64: base64url encoding has no padding

See RFC4648 section 5 and RFC7540 section 3.2.1.

Suppress generation of '=' padding of base64url encoding. This is
accomplished by considering the string beginning at offset 64 in the
character table as the padding: this is "=" for base64, "" for base64url.

Also use strchr() to replace character search loops where possible.

Suppress erroneous comments about empty encoding results.

Adjust unit test 1302 to unpadded base64url encoding and add tests for
empty results.

Closes #9139